Calibration of solar reflectance panel

Calibration is provided for a diffuser panel (30) on board a spacecraft (10) employed in measuring the intensity of sunlight reflected from subjects on the earth. The calibration is accomplished by comparing the intensity of solar radiation reflected from the panel with the intensity of the solar radiation incident directly from the sun. The comparison is obtained by directing both radiation into an integrating sphere (60) through separate ports (62, 64) which are sized to admit substantially equal amounts of power of the reflected and the direct radiations. A detector (76) detects an average value of intensity of the reflected radiation while the direct rays are excluded by a shutter (94). Upon an opening of the shutter and a deflecting of the diffuser panel from the calibrating position, the detector detects the average value of radiation intensity from the direct rays of the sun. The detected values of the radiation intensities may be transmitted back to earth, or are stored in memories 108 and 110 of a computer 100, and then applied to an arithmetic unit 114 to produce the calibration ratio which is then made available for transmission with imaging data back to the earth.